The long term objectives of this application are to better understand the cellular events taking place during the process of bone remodeling. This process is sequential and involves a number of steps and a number of cell types. Among these cell types, the osteoclast is the cell responsible for the extracellular resorption of the bone matrix. This cell is also potentially involved, directly or via its precursors, in the recognition of the matrix to resorb. Similarly, the osteoclasts or their activity if potentially involved in the local activation of bone formation. Most diseases of the skeleton, whether metabolic (osteoporosis, renal osteodystrophy), inherited (osteopetrosis, osteogenesis imperfecta), infectious (Paget, periodontal disease, osteoarthritis) or tumoral in nature, involve an abnormality in the bone remodeling sequence. The osteoclasts playing such an essential role in this process, our research program focuses on the cell and molecular biology of the osteoclast's function and differentiation. The specific aims of this application are: 1/ to further characterize the plasma membrane of the mature osteoclast at the molecular level by: a) further determining the role of the (Na+,K+) ATPase subunits in bone resorption, and b) generating new monoclonal and monospecific polyclonal antibodies, to further investigate the function of this cell. 2/ To better understand the biology of the osteoclast at the cellular level and in particular: a) The mechanisms by which lysosomal enzymes and other potential secretory protein(s) are targeted to the ruffled border membrane: b) The differences between the apical (ruffled-border) and baso- lateral membranes; c) The mechanisms by which the polarity of this cell is established and maintained; d) The role of other ion transport proteins in the mechanisms of the bone resorption. 3/ Further compare the osteoclast to other cells involved in transport and acidification (Gastric parietal cell, renal tubule, hepatocyte) 4/ Further analyse the effects of calcium regulating hormones on the distribution, expression and function of the abovementioned molecules and/or mechanisms in which they are involved. The experimental approach selected in this project involves both in vivo and in vitro studies, at the organ, cellular and subcellular levels and in various species (chicken, rat, rabbit). The methods of procedure include the purification and culture of osteoclasts, the fractionation of their membranes, the development of monoclonal and polyclonal antibodies, the analysis of membrane proteins by SDS-PAGE and immunochemical methods, immunocytochemistry at the light and electron microscopic levels, and the in situ hybridization of selected probes.